1. Field of the Invention
The present invention relates to an optical module 1 that handles three types of optical signals and is usable in the optical CATV system.
2. Related Prior Arts
The PON system (Passive Optical Network system) transmits and receives two types of optical signals, each having wavelengths of 1.31 μm and 1.55 μm through a single optical fiber 50. A bi-directional optical module 1 applicable to this PON system is necessary to provide, in addition to a light receiving device and a light-transmitting device, a WDM (Wavelength Division Multiplexing) coupler or a WDM filer. A Japanese Patent Application published as JP-2005-099482A has disclosed such a bi-directional optical module 1. This optical module 1 has two devices each installing the semiconductor light-receiving device or the semiconductor light-transmitting device.
A United States Patent, the U.S. Pat. No. 7,093,988, has disclosed another type of the bi-directional optical module 1 as one modification of the module above, in which a semiconductor device to receive signal light and another semiconductor device to transmit another signal light are housed within a single package accompanied with the WDM filter. In this modified optical module 1 shown in FIG. 1 of the prior art, within the package comprised of the stem 114 and the cap 110 with the lens is enclosed with the light-transmitting device 113 and the light-receiving device 112. The light emitted from the light-transmitting device 113 couples with the optical fiber 132 after it is reflected by the WDM filter 115 and focused by the lens 116. On the other hand, the light provided from the optical fiber 132 couples with the light-receiving device 112 after it is focused by the lens 116 and passes through the WMD filter 115.
Such a bi-directional module with a single package, when it is installed in an optical communication system, may leave a space in a side of this bi-directional module for another functional device to be installed therein. From the viewpoint of the system configuration, one request to transmit another optical signal in an analog form, in addition to digital optical signals each being transmitted in up-and-down directions, has become practical; in particular, the optical CATV (Cable Television) system has strongly demand this new configuration.
The conventional CATV system that uses a copper cable has transmit analogue video signals, whose frequency range is 100 to 700 MHz, from the center station to respective subscribers by the frequency multiplexing, while, each subscriber has upload digital signals with a frequency range below 50 MHz to the center station. Currently, the optical CATV system first converts the downward video signal into an optical signal with the same frequency spectrum of the analogue electrical signal explained above. In addition, the CATV system now strongly demands to access the internet system optically. Thus, the current CATV system is necessary to handle three types of optical signals, one is the downward video signal and the other two are the upward and the downward digital signals for the internet access.
To satisfy the request in the current CATV system mentioned above, a new architecture has been proposed in which three optical signals are transmitted by multiplexing different wavelengths. The downward video signal assigns 1.55 μm, the downward digital signal assigns 1.49 μm, and the upward digital signal assigns 1.31 μm. The optical module 1 applicable to such a new optical CATV system provides the bi-directional module with the single package that handles the upward and the downward digital signals, and an optical-receiving module for the analog video signal set in the side of the module where the conventional bi-directional module with two or more packages assembles the light-receiving module. This type of the optical module 1 is what is called as tri-plexer.
The analog video signal has a frequency range from 40 to 870 MHz with the frequency multiplexing configuration and the wavelength of 1.55 μm, while, the downward digital signal may have several standards, for instance, a standard whose transmission speed up to 620 Mbps is called as BPON, those up to 1.25 Gbps is called as EPON, and those up to 2.5 Gbps is called as GPON. The present invention relates to this type of the tri-plexer and an optical module 1 installing such a tri-plexer.